In conventional automatic telephone answering apparatus, an OGM which instructs the start of automatic recording to a calling party is recorded on a recording medium (for example, a magnetic tape) in advance. Upon reception of an incoming call, first the OGM is reproduced to be sent to the calling party over the telephone line and, thereafter, an ICM from the calling party can be recorded on the magnetic tape. There are two types of automatic telephone answering apparatus, a first type in which the OGM and the ICM are recorded using a single magnetic tape as the recording medium, and a second type in which the OGM and the ICM are recorded on different magnetic tapes. In either type of apparatus, upon recording and reproducing of the OGM and the ICM, position detection of the magnetic tape, that is, a search for the beginning, an intermediate division and the end of the OGM and/or the ICM, must be performed. For this purpose, a position detection system is employed in which a cue signal is recorded at a predetermined position on the magnetic tape and is reproduced to indicate the respective position.
However, in order to produce the cue signal, prior art apparatus required an exclusive-use cue signal oscillator. In addition, individual signal oscillators were required for producing various sound signals, for example, a beep tone signal for notifying a calling party and a user of the operational state, an operational step, or an operation instruction; a reception sound signal of a key operation, which is produced upon a satisfactory operation of an operation key on a keyboard; an alarm signal which is produced upon malfunction or erroneous operation; and the like. In the case of a push-button or touch tone type telephone set, a touch tone signal is sent onto a telephone line upon input of a desired telephone number by actuation of the respective keys of a keyboard. A touch tone signal oscillator is also required for generating the touch tone signal. Therefore, in the system combining the above-described telephone set and automatic telephone answering apparatus, a plurality of signal oscillators must be provided.
Moreover, the oscillator for providing a cue signal must have a very stable frequency which is not easily influenced by an audio signal. If such an oscillator is not adopted, high precision is required for the characteristics of a cue signal detection circuit. Otherwise the cue signal may not be discriminated from the audio signal and would be easily influenced by induction noise, e.g., humming.